HOW best to describe “Passing Strange,” which strangely passed from downtown’s Public Theater to the uptown Belasco?

Devised by the plump, stately singer/songwriter Stew (real name behind the shades: Mark Stewart) and Heidi Rodewald, is it a pop punk-rock concert without the hoopla – or a lounge act without the lounge?

As a rock concert with a theme, it’s not exactly a show that could play Madison Square Garden, and at about 2½ hours, it clocks in far too long for a lounge act.

Yet, with its bare-bones staging by Annie Dorsen and Stew’s anecdotal book and lyrics, it hardly measures up to a Broadway musical, either – although the producers probably base their hopes on the success of the far more original “Rent.”

“Passing Strange” is more like a Broadway cantata, a recycling of theater song-cycles of the likes Joe Papp encouraged at the Public, and sometimes risked on Broadway, many years ago.

It’s also beautifully performed by a beguiling cast – fun people to be with, even if one has to be with them rather longer than one might have planned.

The “Candide”-like theme tells a picaresque tale of a young man’s quest for self-knowledge and identity. In this case, a middle-class black kid from Los Angeles tries out life in Europe – drugs, music and sex in Amsterdam, and revolution, music and sex in West Berlin – before returning, freshly self-identified, to LA.

In Berlin, as a passport to the pseudo-revolutionary world our hero aspires to, he invents ghetto credentials – or, as the show puts it, “passing strange.”

This is a conceit less strange than the show’s authors try to suggest: Self-invention is often a prelude to self-identification. Yet, for all its conventionality, Stew’s book and particularly his lyrics are witty and pointed.

He has a dry sense of humor that’s perfectly on-target, and stands back from these presumably autobiographical vignettes with a wry but calculated modesty.

The music is very loud but less original, with most of the onstage band – including co-composer Rodewald on bass – joining in the singing.

It’s a great, if virtually unknown, cast, all of whom – except Stew, the show’s narrator, and a brilliant Daniel Breaker, who plays his younger and more agile self – play multiple roles with a vivacity that matches Broadway’s finest.

At the core of it is the altogether engaging Stew. He’s a fine artist, and although Broadway may not be his alley, his offbeat beatness would be a delight to encounter in cabaret.